1. Field of the Invention
The present invention relates to a signal processing apparatus and a signal processing method, and more particularly relates to the signal processing apparatus for and the signal processing method of processing a signal at a time of recording an audio signal onto an information record medium such as an MD (Mini Disc) in an information recording apparatus.
2. Description of the Related Art
In order to compress the information amount or volume for the MD to about one fifth of that of a CD (Compact Disc), an audio data high efficiency encoding method, which is so called as an ATRAC (Adaptive TRansform Acoustic Coding), is employed. The theory of the ATRAC is to perform a compression of the information by removing a redundant portion of the information signal, which are not sensible by a human being with consideration of the auditory sense of the ears of the human being. In other words, on an assumption that a difference between the original signal and the signal after compression is generated (i.e., the quantization noise is generated), the compression is performed while controlling the quantization noise so as not to be audible for the human being.
More concretely, in the ATRAC, the high efficiency encoding method is performed by use of the masking effect and the loudness characteristic related to the property on the auditory sense of the human being (i.e., the minimum audible limit curve). This method is explained with an example. In FIG. 7A, a minimum audible limit curve 100 is indicated. The audio sound at the level below the curve 100 cannot be heard by the ears of the human being. Now, assuming that a sound A and a sound B indicated on the graph of FIG. 7A are included in the input signal, the sound B is audible by the human being but the sound A is not audible by the human being since it is positioned on the graph at a position below the minimum audible limit curve 100. Therefore, in the ATRAC, the sound B is recorded as the effective signal. On the other hand, the sound A is not recorded (thinned out), or is encoded while drastically reducing the quantization bit amount.
In FIG. 7B, in addition to the minimum audible limit curve 100, a masking characteristic curve 110 due to a sound C is indicated. Although the sound B is positioned on the graph at a position above the minimum audible limit curve 100, the sound B cannot be heard by the ears of the human being since it is masked by the sound C positioned at the vicinity of the sound B on the graph. Therefore, in the ATRAC, the sound B is also removed as the redundant component and is then compressed.
As explained above with the example, the signal compressing method in the ATRAC depends on how to set the minimum audible limit curve, the masking characteristic and so on, which are the parameters in the compressing process. Here, the compression encoding process by means of the ATRAC is performed by use of just one set of the minimum audible limit curve, the masking characteristic and so on, which are predetermined in advance.
However, the record signal or the music impression for the human being is quite different depending upon the setting of the minimum audible limit curve and/or the masking characteristic used for performing the compression encoding process by means of the ATRAC. Therefore, if a plurality of music compositions in various music tones or genres are recorded by use of just one fixed set of these characteristics, an optimum compression encoding process cannot be always applied to the original sounds (i.e., the original music composition), so that a case may be raised in which the recording operation suitable for each music composition cannot be performed e.g., the once recorded and reproduced music composition does not give any force or good impression to the audience or the high frequency component of the once recorded and reproduced music composition jars on the ears of the human being.